Research in this application is directed to fundamental problems in the biogenesis and function of membranes. A major objective is to obtain information on the processes by which cells sense the osmolarity (chemical potential of water) in the medium in which they live, and react to variations in osmolarity by appropriate adaptive responses. Osmotic regulation of the volume of the cytosol is a fundamental problem faced by all living cells, arising from the fact that the plasma membrane is freely permeable to water. Its importance in human biology is clearly shown by the elaborate mechanisms developed by mammals to balance the osmolarity of the extracellular fluids with that of the cell contents. This aspect of homeostasis is vital to renal physiology and pathology and for the regulation of electrolyte transport into and out of mammalian cells. Recent work in our own and other laboratories has revealed the importance of periplasmic glucans, including the membrane-derived oligosaccharides (MDO) of Escherichia coli and the closely related cyclic glucans of the Rhizobiaceae in. osmotic adaptation and cell signalling. This application is focussed on the MDO of E. coli with the following specific objectives: (1) The biosynthesis of MDO, (2) The galU gene and its functions, and (3) Feedback inhibition of MDO synthesis.